1. Field of the Invention
The present invention relates generally to cleaning apparatuses, and more specifically, to a cleaning apparatus for cleaning semiconductor wafers with cleaning chemicals.
2. Description of the Background Art
Various steps (such as removal of native oxide films and formation of contact holes) are involved in treating the surface of a semiconductor wafer (hereinafter simply referred to as wafer) in the manufacture of a semiconductor device.
Referring to FIG. 9, a surface-treated wafer 100 is placed in a basket 101, sent together with basket 101 into a cleaning vessel 2 filled with chemicals and cleaned. Then, wafer 100 is sent to water tanks 102 and 103 filled with water and washed with the water. The washed wafer is sent to a drying chamber 104 and dried.
FIG. 10 is a view showing the concept of a conventional wafer cleaning apparatus. The conventional cleaning apparatus includes a cleaning chemicals tank 1 storing cleaning chemicals. NH.sub.3 +H.sub.2 O.sub.2, HCl+H.sub.2 O.sub.2, HF or the like is used for such cleaning chemicals. A cleaning vessel 2 accommodates the cleaning chemicals supplied from cleaning chemicals tank 1. New cleaning chemicals is supplied into cleaning vessel 2 from cleaning chemicals tank 1 through a cleaning chemicals supply conduit 3. A supply valve 4 is provided in cleaning chemicals supply conduit 3 for regulating the amount of cleaning chemicals flowing in cleaning vessel supply conduit 3. A conduit for waste fluid 5 for externally discharging used cleaning chemicals is connected to the bottom of cleaning vessel 2. Conduit for waste fluid 5 is provided with a waste fluid valve 6 for regulating the amount of used cleaning chemicals flowing in waste fluid conduit 5. The cleaning chemicals in cleaning vessel 2 is circulated by a pump 48. Foreign matters in the chemicals are removed away by a filter 49 during the circulation.
Cleaning vessel 2 must have resistance to chemicals and heat, strength and the like. Cleaning vessel 2 is therefore formed of a teflon based material (such as PTFE and PFA), SUS, silica, glass or the like. When acidic or alkaline chemicals is used for the cleaning chemicals, cleaning vessel 2 is mostly formed of a teflon based material. The cleaning vessel is sometimes partially formed of silica or glass in such a case, but it is undesirable to use such a fragile material which might crack. The above-described teflon based material is hydrophobic, in other words does not unite with water.
Now, a cleaning operation will be described.
By closing waste fluid valve 6 and opening supply valve 4, new cleaning chemicals is introduced from cleaning chemicals tank 1 into cleaning vessel 2. About 30 liter of cleaning chemicals is accommodated in cleaning vessel 2. About 25 wafers are introduced into cleaning vessel 2 and cleaned. Organic substances, foreign matters, metals, etc. sticking to the surfaces of the wafers are removed away by the cleaning.
The cleaning process is completed by taking out the wafers from cleaning vessel 2.
When such a cleaning process is repeated a number of times, the cleaning chemicals in cleaning vessel 2 is contaminated, and therefore, the chemicals must be exchanged. The present invention relates to exchanging the cleaning chemicals.
First how cleaning chemicals is exchanged in a conventional cleaning vessel will be described.
Supply valve 4 is closed, and waste fluid valve 6 is opened. Used cleaning chemicals is discharged from vellel 2 through waste fluid conduit 5. When the used cleaning chemicals is entirely discharged from cleaning vessel 2, waste fluid valve 6 is closed and supply valve 4 is opened. New cleaning chemicals is guided into cleaning vessel 2 through cleaning chemicals supply conduit 3, thus completing the operation of exchanging the cleaning chemicals.
The conventional exchanging operation for cleaning chemicals in the cleaning vessel was very simple as described above. The conventional cleaning vessel is however disadvantageous when particles about as large as 0.1 .mu.m in size present in cleaning chemicals are addressed. More specifically, in a cleaning vessel for semiconductor wafer, particles contained in used cleaning chemicals stick to the inner surface of cleaning vessel 2 at the time of exchanging the cleaning chemicals, or foreign matters may be produced from the inner surface of cleaning vessel 2 itself when it dries. Since a teflon based material is hydrophobic, and therefore foreign matters are more likely to be produced when the wall face dries. As a result, even if new cleaning chemicals without any such particles is supplied into cleaning vessel 2, particles already sticking to the inner surface of cleaning vessel 2 or foreign matters produced from the inner wall surface itself is taken into the new cleaning chemicals, resulting in contamination of the new cleaning chemicals. Such a problem is hardly recognized when foreign matters about as large as 0.3 .mu.m in particle size are objects to be removed, but it cannot be ignored when foreign matters about as large as 0.1 .mu.m must be removed.
FIG. 11 is a graph showing a result of measurement of the number of foreign matters in cleaning chemicals immediately after and before exchange. The same test was conducted twice (Sample 1 and Sample 2). FIG. 12 is a chart showing the relation between time passed from immediately after exchange of the cleaning chemicals and the number of foreign matters. Referring to these figures, the number of foreign matters in the cleaning chemicals immediately before exchange is smaller than the number of foreign matters in the cleaning chemicals immediately after exchange, because before exchanging the cleaning chemicals, the cleaning chemicals is filtered and circulated during cleaning. The cleaning chemicals immediately after exchange contains more foreign matters, because particles stick to the inner wall surface of the cleaning vessel or the inner wall surface of the cleaning vessel dries to produce foreign matters as described above, and they are mixed into new cleaning chemicals.
One method of reducing foreign matters in cleaning chemicals immediately after exchanging cleaning chemicals is to once fill cleaning vessel 2 with new cleaning chemicals, then discharge the chemicals, and once again introduce new cleaning chemicals into cleaning vessel 2, but this method disadvantageously uses a large amount of chemicals.